The present invention relates to a polyurethane resin, a process for preparing the same and a printing ink composition for a laminate containing the same. More particularly, it relates to a polyurethane resin useful as a binder resin for a printing ink for a laminate or the like which exhibits excellent adhesiveness for various kinds of plastic films and blocking resistance, and excellent adaptability for boiling or retorting treatment, a process for preparing the same and a printing ink for a laminate containing the same.
Recent diversification of package bags or containers has required a high degree of performance for printing inks or coating agents used for the ornamentation or surface protection thereof.
For instance, printing inks for plastic films are needed to provide more excellent printability, adhesion to a wider range of films, more excellent blocking resistance and gloss than conventional ones.
Especially, in the field of food packaging, bags or containers made of laminated film materials are used for the reasons that they are sanitary because the content thereof does not come in direct contact with the ink and that they provide a satisfactory appearance as a high grade of printed products.
Generally there are the following two methods for producing such laminated film materials: The one is an extrusion laminating method wherein a plastic film as a printing substrate is printed with an ink, and if necessary, a primer is applied onto the inked surface, and a molten resin such as polyolefin is extruded onto the inked surface. The other is an adhesive laminating method wherein an adhesive is applied onto the inked surface mentioned above, and a plastic film is laminated onto the surface. Accordingly, the laminating inks must be excellent in adhesiveness to the printing substrate as well as in adhesiveness (lamination strength) to a film to be laminated.
In the case of the laminated film materials which further undergo a boiling or retorting treatment wherein the packages made thereof are immersed in hot water for the purpose of cooking or sterilization of the content therein, they are needed to have an adaptability for boiling or retorting treatment which means the performance of preventing delamination through the boiling or retorting treatment.
Most of these performances required for the printing ink mainly depend upon the performance of a resin used as the binder of the ink. Accordingly, in the case of printing inks for plastic film, one or more of various binder resins are selected so as to meet the performance required for individual printing inks.
In general, however, when a resin which hardens an ink film is used as a binder, there is a tendency that the resulting ink film provides blocking resistance but is reduced in adhesion to plastic film and in lamination strength. For instance, in the case of binder resins such as acrylic resins, polyamide resins and polyester resins, an aromatic ring or a highly polar functional group is introduced in the molecule to harden the resin film. However, these means invite the reduction of adhesion and lamination strength. Accordingly, those resins are unsuitable as a binder for an ink to be applied to a wide variety of films or a laminating ink.
On the other hand, polyurethane resins can be improved both in hardness of the coating film thereof and in adhesion to plastic films by increasing the concentration of urethane bond the molecule. An ink containing a polyurethane resin of this type as a binder provides good adhesiveness and lamination strength, as compared with an ink containing other resin which provides an ink film having the same hardness. For the reason, polyurethane resins wherein the concentration of urethane bond is increased as highly as possible have been used as the binder for printing inks to be applied to laminates of plastic films.
However, a polyurethane resin wherein the concentration of urethane bond is increased highly is poor in adaptability for boiling or retorting treatment, which results in failure to give to a printing ink a sufficient adaptability for boiling or retorting treatment. For the purpose of eliminating this problem of insufficient adaptability for boiling or retorting treatment, there is used a two-component reaction type printing ink composed of one component containing a polyurethane resin and the other component containing a polyisocyanate compound as a curing agent.
However, the two-component reaction type printing ink has various problems as follows: Since the main component must be mixed with the curing agent immediately before printing, the ink is inconvenient in handling, the pot life is short and the remaining ink after printing is unstable. Further, the use of expensive curing agent raises costs of packaging bags or containers.
It is an object of the present invention to provide a polyurethane resin useful as a binder resin for use in a printing ink for laminate or the like.
Another object of the present invention to provide a process for preparing the polyurethane resin.
Still another object of the present invention is to provide a printing ink for a laminate containing the polyurethane resin.
These and other objects of the present invention will become apparent from the description hereinafter.